This invention relates to spread-spectrum communications, and more particularly to code-division-multiple-access (CDMA) cellular, packet-switched communication systems. The inventive concepts involve optimization of packet data communications using a hybrid DSMA-CR/CDMA multiple access method with collision resolution.
Recent developments in wireless communications technologies have allowed expansion of service offerings from the original voice telephone service model to include a number of services supporting packet data communications. As customers become increasingly familiar with data services offered through landline networks, they are increasingly demanding comparable data communications in the wireless domain, for example to maintain service while mobile subscribers roam freely or to provide remote service in locations where wireless loops are preferable to landline subscriber loops. A number of technologies support packet data communications in the wireless domain.
For example, a common-packet channel (CPCH) is an uplink transport channel for transmitting variable size packets from a mobile station (MS) to a base station (BS) without the need for direct resource allocation. The channel resource allocation is contention based. A number of mobile stations could at any time content for the same resources, as found in ALOHA systems. The basic CPCH packet data communication does not address issues such as assignment of the channel resources to various base stations, broadcasting channel status and data rates by the base stations and piggy-backing. If a mobile station cannot access one channel, because the channel is busy, it tries another channel, and so on until it finds and accesses an available channel. When many users are trying to access a limited number of channels, the failed access attempts alone can impose a substantial load and even possibly overload one or more channels of the system.
In a Digital Sense Multiple Access (DSMA) system, whenever a base station detects the presence of a subscriber unit transmission on the reverse channel it asserts a periodically occurring flag, called a xe2x80x9cbusy/idlexe2x80x9d flag, on the associated forward channel. This flag is asserted logically true whenever the channel is busy. Any subscriber unit that already is transmitting when the busy/idle flag is set true may continue to transmit. However, all other subscriber units desiring access to a channel must wait until the busy/idle flag is reset or cleared indicating that at least one channel is idle and available. Since the two-way propagation delay is much less than the minimum packet length, the DSMA type protocols perform much better than traditional slotted ALOHA type protocols. The physical layer and the underlying spread spectrum system allow the quick detection of a collision. This approach only allows the base station to generally throttle back the traffic flow. Essentially, each of the mobile stations will interpret a busy state as an instruction to xe2x80x9cbackoffxe2x80x9d and delay its next access attempt. Although this control from the base station helps to prevent overload, it does not work well when there are multiple CPCH channels. The mobile station(s) cannot determine the busy or idle status of multiple channels from a single busy/idle flag signal.
This invention introduces a hybrid DSMA-CR/CDMA method and apparatus, which addresses the above issues, thus accommodating bursty packet data traffic in an optimum manner. The hybrid DSMA-CR/CDMA method provides a proactive approach to avoid collision by having the base station broadcast the availability and available data rates for each of its channels or for each group of its channels. The mobile station can use the broadcast information to select an idle channel with sufficient data rate before its transmission, instead of waiting for the base station to resolve the collision, if any, in later procedures. The method also allows priority schemes.
Hence a general objective of the invention is to allow mobile stations to assess the availability of packet channel resources.
Another objective relates to efficiently assigning limited channel resources to various base stations.
A further objective is to allow the mobile units to efficiently utilize the acquired channel by continuously transmitting additional packets, which have been passed on from higher layers during transmission.
The present invention provides an improvement to a code-division-multiple-access (CDMA) system employing spread-spectrum modulation. The CDMA system has a radio network controller (RNC) and a plurality of base stations, which serve a plurality of mobile or remote stations. Each base station has a BS-spread-spectrum transmitter and a BS-spread- spectrum receiver. Each of the mobile stations has an MS-spread-spectrum transmitter and an MS-spread-spectrum receiver. In the preferred embodiment, the RNC monitors traffic demand, based on traffic measurement information of communications through the base stations for the mobile stations. Based on the traffic demand or a projection thereof, the RNC assigns channel resources to the base stations, by re-configuring the channel resources within each cell.
At a base station, the inventive method involves broadcasting, on a periodic or non- periodic basis, availability related status information of one or more of the channels allocated to the base station. For example, the status information can contain actual availability information, i.e. idle or busy, or available data rate information, or both. At a mobile station, the steps include monitoring the broadcast(s) of the status information. Based on the broadcast status information, the mobile station selects an idle channel. The channel selection by the mobile station can utilize a dynamic persistence algorithm or any other algorithm commonly known in the art.
Following channel selection, the mobile station starts transmission of a series of access preambles. Each preamble contains a signature selected from a set of predefined signatures used for communications with the base station. Typically, the preamble signature corresponds to the spreading code and/or the scrambling code used by the network to define the selected logical channel at the physical layer of the CDMA network. The mobile station transmits the preambles, at well-selected time intervals and at increasing power levels. The mobile station stops its transmission of the access preambles when the access preamble has been picked up and detected by the base station, the base station has responded with an acknowledgment AP-AICH, and the mobile station has also successfully received the AP-AICH. Alternatively, the mobile station ceases its access preamble transmissions if the mobile station has transmitted the maximum allowed number of access preambles MAP.
Upon receiving this AP-AICH signal, the mobile station randomly selects a collision detection (CD) signature and transmits a CD preamble containing that signature. This CD preamble procedure is used to prevent interference when more than one MS has received an AP-AICH from the BS. When the base station receives CD preambles, it selects and identifies one preamble. The base station, then transmits a CD acknowledgement channel (CD-AICH) signal corresponding to the CD preamble selected from the received signals. Assuming successful resolution of any overlaps, and selection of the one CD preamble, the base station has effectively assigned the CPCH channel to the one mobile station. Upon receiving a CD acknowledgement CD-AICH, which correspond to the sent CD signature, the mobile station begins to send its packet data along with any closed-loop power control information. The base station also sends its downlink closed-loop power control information simultaneously. A pre-data power control phase is optional.
If the MS detects a loss of the downlink channel, for example, during transmission of the power control preamble or the packet data, the MS halts CPCH uplink transmission. Essentially, the mobile station aborts the access attempt and sends a failure message to the MAC layer of the associated data equipment. The base station can utilize this feature, by cutting off the downlink transmission, to instruct mobile stations not to use a channel following an unresolved collision.
During a transmission of data, the mobile station that has successfully obtained access can piggy-back packets, one after another, so long as it has packets ready to send, up to a maximum limit set by the network. Essentially, this allows the mobile station to hold the CPCH channel if the MAC equipment supplies further packets to the PHY elements in the midst of an uplink transmission.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following and the accompanying drawings or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.